ALL DESIGN TOOLS
What isn't a compositor might be a better question to ask but let's keep this article on point. In short a compositor is responsible for fusing together elements such as live action footage, 3d animation, stock footage and other sources into a single picture. Sometimes the goal is photo-realism for a motion picture and other times the goal is to exaggerate the world for a TV spot.
A good comparison might be the job of a weldor. Wikipedia has a curious description that I found particularly useful:
"Welders typically have to have good dexterity and attention to detail, as well as some technical knowledge about the materials being joined and best practices in the field."
In order to complete some shots, a compositor might take on other responsibilities such as modeling and rendering a 3D object or element. Similarly, a weldor might need to fabricate a special tool or object needed for a custom vehicle manipulation.
When a weldor is combining various parts of a bicycle frame, it is important that the parts are fused together properly or the bicycle might just fall apart. Much like a visual effects shot that is not sound, you may see it fall apart on screen.
Knowing the tools and understanding the related jobs of this industry might just answer questions you didn't know you had. Never stop learning and developing new techniques.
Wikipedia also has a note regarding weldors:
"Welders are also often exposed to dangerous gases and particulate matter."
But I won't get into that.
